MEMS devices are becoming increasingly popular. MEMS transducers, and especially MEMS capacitive microphones, are increasingly being used in portable electronic devices such as mobile telephone and portable computing devices.
Microphone devices formed using MEMS fabrication processes typically comprise one or more moveable membranes and a static backplate, with a respective electrode deposited on the membrane(s) and backplate, wherein one electrode is used for read-out/drive and the other is used for biasing. A substrate supports at least the membrane(s) and typically the backplate also. In the case of MEMS pressure sensors and microphones the read out is usually accomplished by measuring the capacitance between the membrane and backplate electrodes. In the case of transducers, the device is driven, i.e. biased, by a potential difference provided across the membrane and backplate electrodes.
FIGS. 1a and 1b provide a schematic diagram and a perspective view, respectively, of a known capacitive MEMS microphone device 10. The capacitive microphone device 10 comprises a membrane layer 11 which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode 3 is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 2 is mechanically coupled to a generally rigid structural layer or backplate 4, which together form a second capacitive plate of the capacitive microphone device. In the example shown in FIG. 1a the second electrode 2 is embedded within the backplate structure 4.
The capacitive microphone is formed on a substrate 101, for example a silicon wafer, which may have upper and lower oxide layers 105, 103 formed thereon. A cavity or through-hole 8 in the substrate and in any overlying layers (hereinafter also referred to as a substrate cavity) is provided below the membrane, and may be formed for example using a “back-etch” through the substrate 101. The substrate cavity 8 connects to a first cavity 9 located directly below the membrane. These cavities 8 and 9 may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes 2 and 3 is a second cavity 10. A plurality of bleed holes 11 connect the first cavity 9 and the second cavity 10.
A plurality of acoustic holes 12 are arranged in the back-plate 4 so as to allow free movement of air molecules through the back plate, such that the second cavity 10 forms part of an acoustic volume with a space on the other side of the back-plate. The membrane 11 is thus supported between two volumes, one volume comprising cavities 9 and substrate cavity 8 and another volume comprising cavity 11 and any space above the back-plate. These volumes are sized such that the membrane can move in response to the sound waves entering via one of these volumes. Typically the volume through which incident sound waves reach the membrane is termed the “front volume” with the other volume, which may be substantially sealed, being referred to as a “back volume”.
In some applications the backplate may be arranged in the front volume, so that incident sound reaches the membrane via the acoustic holes 12 in the backplate 4. In such a case the substrate cavity 8 may be sized to provide at least a significant part of a suitable back-volume. In other applications, the microphone may be arranged so that sound may be received via the substrate cavity 8 in use, i.e. the substrate cavity forms part of an acoustic channel to the membrane and part of the front volume. In such applications the backplate 4 forms part of the back-volume which is typically enclosed by some other structure, such as a suitable package.
It should also be noted that whilst FIGS. 1a and 1b show the backplate 4 being supported on the opposite side of the membrane to the substrate 101, arrangements are known where the backplate is formed closest to the substrate with the membrane layer 11 supported above it.
In use, in response to a sound wave corresponding to a pressure wave incident on a MEMS microphone transducer, the membrane is deformed slightly from its equilibrium position. The distance between the lower electrode 3 and the upper electrode 2 is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown).
To provide protection the MEMS transducer will typically be contained within a package. The package effectively encloses the MEMS transducer and can provide environmental protection and may also provide shielding for electromagnetic interference (EMI) or the like. The package also provides at least one external connection for outputting the electrical signal to downstream circuitry. For microphones and the like the package will typically have a sound port to allow transmission of sound waves to/from the transducer within the package and the transducer may be configured so that the flexible membrane is located between first and second volumes, i.e. spaces/cavities that may be filled with air (or some other gas suitable for transmission of acoustic waves), and which are sized sufficiently so that the transducer provides the desired acoustic response. The sound port acoustically couples to a first volume on one side of the transducer membrane, which may sometimes be referred to as a front volume. The second volume, sometimes referred to as a back volume, on the other side of the one of more membranes is generally required to allow the membrane to move freely in response to incident sound or pressure waves, and this back volume may be substantially sealed (although it will be appreciated by one skilled in the art that for MEMS microphones and the like the first and second volumes may be connected by one or more flow paths, such as small holes in the membrane, that are configured so as present a relatively high acoustic impedance at the desired acoustic frequencies but which allow for low-frequency pressure equalisation between the two volumes to account for pressure differentials due to temperature changes or the like).
Various package designs are known. For example, FIGS. 2a and 2b illustrate “lid-type” packages. A MEMS transducer 200 is mounted to an upper surface of a package substrate 201. The package substrate 201 may be PCB (printed circuit board), i.e. FR4, or any other suitable material. A cover or “lid” 209 is typically located over the transducer 200 and is attached to the upper surface of the package substrate 201. The cover 209 may, for example, be a metallic lid. An aperture 208 in the substrate 201 provides a sound port and the flexible membrane of the transducer extends over the sound port.
The package will typically also contain electrical circuitry 250, customised for a particular application, which may be integrated with the MEMS die as shown in FIG. 2b or provided separately as shown in FIG. 2a. The integrated circuit may provide bias to the transducer and may buffer or amplify a signal from the transducer. According to convention, the configuration shown in FIGS. 2a and 2b—in which the sound port 208 is provided on same side of the package to the external electrical connection—is known as a “bottom port” configuration. It will be appreciated that the term “bottom port” does not imply any particular orientation of the package device either during manufacture, processing or any subsequent application.
As a consequence of the need for a port hole, i.e. an acoustic port, within the package, environmental contamination in the form of, for example, solid particles, within the package will still arise. It will be appreciated that the presence of solid particles within any of the holes/cavities etc. of a MEMS transducer, in particular in the air gap between the two electrodes, i.e. between the backplate and the membrane, may have a detrimental effect on the performance and/or functionality of the device. For example, in the case of a MEMS microphone transducer, solid particles may enter the transducer package through the acoustic port, i.e. port hole, and may ultimately contaminate the transducer device via the substrate cavity. The occurrence of debris, i.e. particulate matter, within a MEMS transducer, and particularly within the acoustic apertures/cavities of a microphone transducer, may result in a number of problems including the occurrence of electrical shorts, mechanical blockage, i.e. occlusion, and corrosion. Thus, the performance of the MEMS may degrade over time as a result of particle contaminants and critical failure of the device is also possible if the movement of the membrane is inhibited by contaminant particles.
It will also be appreciated that water may enter the transducer package through the port hole. The presence of water within the MEMS cavities may also potentially affect transducer performance and/or functionality, e.g. by shorting out the electrodes and thus rendering the microphone inoperable.
It is known to provide a MEMS transducer with an environmental or ingress barrier, e.g. a particle filter for preventing, or at least inhibiting, particles from reaching the transducer. The environmental barrier, i.e. ingress barrier, may also prevent liquids, e.g. water, from reaching the transducer. However, such a filter or barrier also affects the functioning of the MEMS transducer as these barriers may interrupt or interfere with the transmission of pressure differentials incident on the transducer, as well as affect the transducer performance by, e.g., adding noise.
Examples described herein are concerned with MEMS transducers which incorporate an environmental filter in order to alleviate the problems associated with solid particle contamination and/or the ingress of liquids into the orifices/cavities of the transducer. Examples described herein are particularly, but not exclusively applicable to MEMS microphone transducers.